The present invention relates to a synthetic fiber rope, preferably of aromatic polyamide, to be driven by a rope sheave.
In materials handling technology, especially on elevators, in crane construction, and in open-pit mining, moving ropes are an important element of machinery and subject to heavy use. An especially complex aspect is the loading of driven ropes, for example as they are used in elevator construction.
In conventional elevator installations, the car and supporting sling, which are moved in an elevator hoistway, and a counterweight are connected together by several steel stranded ropes. To raise and lower the car and the counterweight, the ropes run over a traction sheave that is driven by a drive motor. The drive torque is transferred by friction to the section of rope which at any moment is lying in the angle of wrap. At this point the ropes are subjected to tensile, bending, pressure, and torsion stresses. The relative movements caused by bending over the rope sheave cause friction within the rope structure which affects rope wear negatively depending on the concentration of lubricant. Depending on the rope construction, radius of bending, groove profile, and rope safety factor the primary and secondary stresses arising have a negative effect on the condition of the rope.
On elevator installations, as well as the strength requirements, considerations of energy lead to the demand for the smallest possible masses. High-tensile synthetic fiber ropes, for example of aromatic polyamides or aramides with highly oriented molecule chains, fulfill these requirements better than steel ropes.
By comparison with conventional steel ropes of the same cross sectional area and same lifting capacity, ropes constructed of aramide fibers have only between one quarter and one fifth of the specific rope weight. In contrast to steel, due to the uniform alignment of the molecular chains, aramide fibers have a substantially lower transverse strength in relation to their longitudinal load-bearing capacity.
For this reason, to minimize the lateral stresses to which the aramide fibers are subjected as they pass over the traction sheave, there is proposed in EP 0 672 781 A1 an aramide fiber stranded rope with suitable parallel lay for use as a driving rope. The aramide rope which has thereby become known affords very satisfactory values in relation to service life, high wear resistance, and fatigue strength under reverse bending stresses; however, under unfavorable conditions with parallel laid aramide ropes, there is the possibility that partial unwinding of the rope occurs, which is permanently detrimental to the original balance of the rope structure. Such unwinding results firstly from the internal torques around the longitudinal axis of the rope which, depending on the load on the rope, generate unwinding of the rope, and secondly from the rope deflections caused externally, for example by the rope running out of alignment over rope pulleys. In this case, dragging of the rope on the flanks of the grooves causes a further change in the structure of the rope. The unwinding causes excessive lengths in the covering layer of strands which are permanently displaced in one direction or the other depending on the direction of rolling. Such occurrences are undesirable because the functionality of the aramide rope can be permanently impaired.
The present invention concerns a synthetic fiber twin rope to be driven by a rope sheave including: a first load-bearing rope formed of a plurality of synthetic fiber strands able to withstand tension which are twisted in one direction of twist; a second load-bearing rope formed of a plurality of synthetic fiber strands able to withstand tension which are twisted in an opposite direction of twist to the one direction of twist, the second rope being positioned parallel to the first rope at a predetermined distance; and a one piece rope sheath surrounding the first and second ropes to prevent rotation thereof when running over a drive sheave.
The rope according to the present invention has the objective of avoiding the disadvantages of the known synthetic fiber ropes, and proposing a synthetic fiber rope with a construction that is neutral to twisting.
In the twin rope according to the present invention, the rope sheath is formed over aramide fiber ropes and acts as a torque neutralizer. The aramide ropes preferably have an identical rope construction, but the directions of their lays are mirror images of each other, i.e. one rope is right-handed, the other rope is left-handed. This ensures that the opposing torques around the longitudinal rope axis which arise under tension, and when passing over rope sheaves, are mutually compensated by means of the torque neutralizer so that the sum of the torques resulting from the right-hand and left-hand aramide ropes when under load is zero. The external torque acting on the rope as it passes over the traction sheave is neutralized by the external contour of the sheathed twin rope. The former round shape of the rope is now approximately oval, the aramnide rope being preferably twice as wide as high.
The rope construction of each of the twin ropes may then differ from each other if the function of the twin rope in its entirety, meaning the neutralization of the sum of the torques, is given.
The service life of parallel laid strands can be increased if, for example, in the case of two layers laid with parallel lay, the direction of twist of the fibers of the strands of one layer of strands is opposite to the direction of twist of the fibers of strands of the other layer of strands.